Find the divergence of $F=-(x,y)/r^2$ using the definition

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Find the divergence of $F=-(x,y)/r^2$, where $r=\sqrt{x^2+y^2}$. Don't use this formula $\operatorname{div} F=Px+Qy$ where $F= (P,Q)$; I am not familiar with the definition of divergence.

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Assuming you mean $\vec{F} = \frac{-x}{\sqrt{x^2 + y^2}} \hat i + \frac{-y}{\sqrt{x^2 + y^2}} \hat j$

div$\vec{F} = \nabla \cdot \vec{F} = \frac{\partial}{\partial x}\vec{F} + \frac{\partial}{\partial y}\vec{F}$.

Then we have div$\vec{F} = \frac{-y^2}{(x^2 + y^2)^{3/2}} + \frac{-x^2}{(x^2 + y^2)^{3/2}} = - \frac{x^2 + y^2}{(x^2 + y^2)^{3/2}} = - \frac{1}{\sqrt{x^2+y^2}} = -\frac{1}{r}$.